The present invention relates generally to injection molding, and more particularly to a novel method and apparatus for controlling the injection of molten material such as molten metal into a die cavity under pressure from a shot cylinder so as to eliminate undesirable pressure spikes while effecting high pressure during curing of the molten material.
It is well recognized in the art of injection molding, such as in die casting machines or apparatus which employ shot cylinders to inject "shots" of molten material into a die cavity, that controlling the speed of the shot cylinder piston during injection of molten material into the die cavity is critical to preventing pressure spikes or peak pressures which may exceed the die clamping force capacity of the machine and cause separation of the die halves. The pressure spikes may result from both excessive fluid pressure applied to the shot cylinder and the inertia forces created by the shot cylinder piston and associated piston and plunger rods. Separation of the die halves can lead to casting fins being formed on the casting and tolerance ranges being exceeded, thereby leading to unacceptable castings with attendant waste and economic loss.
U.S. Pat. No. 4,066,189 discloses apparatus for controlling injection of molten material under pressure into a die cavity, and attempts to overcome the problem of peak pressure within the die cavity by decreasing the speed of the shot cylinder piston before the mold cavity is completely filled. A relatively complex fluid pressure system provides a run-around circuit between the rod end of the shot cylinder and the head end thereof, and includes means for blocking the flow path through the run-around circuit in response to advance of the shot cylinder piston, whereafter advance of the shot piston is controlled by controlling release of fluid pressure from the rod end of the shot cylinder.
U.S. Pat. No. 4,488,589 discloses a shot cylinder controller for controlling the speed of a shot cylinder so as to compensate for changes in operating variables resulting from changes in the fluid mechanics of the system. The shot cylinder controller employs a relatively complex servo-control system which includes a preselected computer program to control the shot cylinder.
A significant drawback with the known prior apparatus for controlling shot cylinders in die casting machines lies in their reliance on relatively complex arrangements to control the shot cylinder so as to eliminate or substantially reduce pressure spikes during injection of molten material into the die cavity, while effecting rapid pressure buildup within the die cavity during solidification and curing of the molten material.